1. Field of the Invention
This invention relates to an improved quartz tube adapted for thermal processing of semiconductor substrates, and more particularly to a quartz tube comprising a double-layered central tube.
2. The Prior Art
Semiconductor substrates are subjected to high-temperature processing, such as oxidation, evaporation, chemical vapor deposition, diffusion of impurities etc. Transparent fused quartz tubes have been conventionally used in these types of processing, because they are not porous and can be produced from highly pure silica and are not easily contaminated with undesired impurities.
However, fused quartz tubes have a disadvantage in that they can not be produced perfectly symmetrical with the outer diameter maintained at a desired length. Therefore, it is necessary to make a relatively large allowance for the gap between the outer surface of the tube and the furnace, which leads to lowering its thermal efficiency. In addition, the unevenness on the surface of the tube results in a so-called lens effect when ultra-red rays irradiate, i.e., an extraordinarily high temperature appears in a spot within the processing tube and, consequently, undesired reactions occur in the processing of semiconductor substrates. Furthermore, the thickness of the wall of a fused quartz tube is usually limited to from about 3 to about 4 mm, which lowers its thermal capacitance. Then, a liner tube must be provided in the gap between the processing tube and the heater, so as to ensure uniform heating of the tube. Such a tube increases the bulk of the processing apparatus as a whole and, consequently, consumption of power necessary for heating is increased.
The fused quartz tube has another disadvantage, i.e., it deforms at a temperature above 1200.degree. C. On the other hand, thermal processing of semiconductor substrates is often carried out at a temperature of from 1150.degree. to 1300.degree. C. Thus, a fused quartz tube suffers from inevitable deformation during thermal processing. Therefore, quartz susceptors in the tube deform and also semiconductor substrates loaded on them deform and even crack. In addition, the deformed quartz tube suffers from wear due to friction with the quartz susceptors. When they are inserted into the tube and removed from the tube, fine powder of quartz is produced by the friction and contaminates the surface of the substrates. Such a tube must be replaced before unacceptable deformation occurs. Thus, a fused quartz tube has a short service life.
Japanese Patent Application Laid-open No. 51-79581 discloses a fused quartz tube, the central portion of which has a thickness greater than the end portions of the tube. However, this quartz tube still has a tendency to easily deform.